New York University
School of Continuing and Professional Studies
Division of Programs in Information Technology
Introduction to Python
Homework, Session 2
Procedure to follow when writing exercise solutions:
1. The exercises are based entirely on material from the slides.
2. Do not attempt to google a solution. If stuck, refer to the exercise solution as a guide,
make sure it is clear to you, then try to recode from scratch.
3. Be completely clear on your code -- clarity means understanding the type and value of
every object in the code, and what led up to its type and value.
4. Test as you write -- insert print statements that make each object type and value clear.
(Be careful not to let your output jumble together and create more confusion.)
5. Remember that the primary goal is to understand how Python thinks, not just to make
a solution work -- so hacking together or guessing your way to a solution subverts the
purpose of this effort.
Procedure to follow when writing homework solutions:
1. The homework solutions are made up of techniques explored in the exercises.
2. Consider the overall objectives of the script and the steps you might take. Reflect on
what you learned in the slides and exercises. Try to visualize in your mind how the
data should be processed -- what objects would hold what values, how they would
need to be converted or changed. This may not be easy at first, but it should be
attempted. Eventually you'll learn how to think this way.
3. Write an outline, in # comments, of the steps the script will take, without writing any
code. If you really feel at a loss as to what steps to take, feel free to review the
"homework discussion" document, which includes an outline. (Please do remove
comments before submitting.)
4. Referring closely to the reference exampes for the features you think you will need,
compose code to satisfy the first step in the outline.
5. Test the code by printing the resulting object(s) and their type(s).
6. Don't proceed to the next step without testing the first step! Sometimes seeing
the outcome of the first step can give you clues as to the proper next step, or cause
you to modify your outline.
7. If you're not sure what to do, please email me before attempting to search on google.
Google searches are problematic for beginners. We want to learn how to develop a
strategy, not try to apply code solutions found on the web, which in the end will only
take you so far -- that is, not very far in really understanding how to code.
8. Before submitting, review the Code Quality requirements and conform your code to
proper style and code organization. I will quickly return solutions that ignore CQ. :)
EXERCISES
Ex. 2.1 Write a program that takes user input with input() and prints whatever the user
typed.
Sample output:
please enter some text: hey what up
you just wrote: "hey what up"
Ex. 2.2 Again using input(), request the user to enter a number and test to see whether it is
all digits (hint: use the str isdigit() method in an if test). Simply print a success
message if all digits, a failure message if no. (Remember that if can be used with
else to have Python do one thing or another depending on the result of the test.)
Sample program runs:
$ python ex2.3.py
please enter an integer:
that was NOT an integer!
hello
$ python ex2.3.py
please enter an integer:
that was NOT an integer!
55.5
$ python ex2.3.py
please enter an integer:
that was NOT an integer!
"55"
$ python ex2.3.py
please enter an integer:
THANKS FOR THE INTEGER!
55
Ex. 2.3 Based on the above program, place your whole program inside a while True: block.
If the input is all digits (i.e., an integer), include the print statement indicating
success, and follow it with a break statement to leave the loop. As you know, if you
do not break, at the end of the block while's "automatic return" will return to the top
of the block automatically and your block will take input() again.
Sample program runs:
$ python ex2.4.py
please enter an integer:
that was NOT an integer!
please enter an integer:
that was NOT an integer!
please enter an integer:
THANKS FOR THE INTEGER!
hello
one?
5
Ex. 2.4 Use a while loop to count from 1 to 10. Before the loop begins, initialize an integer
counter (count) to 1. The while test should test to see if the counter is less than or
equal to 10. Inside the loop, "increment" count with count = count + 1 (i.e., it will
be incremented upon each iteration of the loop). Print each new value of the
integer. (Note that the slides contain this solution, so your job is to be able to code a
program like this from scratch. Repeat it until it is second nature if you can!)
Expected output:
1
2
3
4
5
6
7
8
9
10
Ex. 2.5 Similar to the above program, use a while loop and a counter to print "Happy
birthday to you!" 10 times (do not print the integer count in this version).
Expected output:
Happy
Happy
Happy
Happy
Happy
Happy
Happy
Happy
Happy
Happy
birthday
birthday
birthday
birthday
birthday
birthday
birthday
birthday
birthday
birthday
to
to
to
to
to
to
to
to
to
to
you!
you!
you!
you!
you!
you!
you!
you!
you!
you!
Ex. 2.6 Modify the above program to take user input with input() and print the message that
many times. (Hint: since we know that input() returns a string, the value will have
to be converted to an int before it can be used in the while test with the loop count
value.)
Sample program run:
$ python ex2.7.py
how many times should I greet you?
Happy birthday to you!
Happy birthday to you!
Happy birthday to you!
3
Ex. 2.7 Modify the above program to test the user input to make sure it is a usable integer.
(Hint: as we did earlier, use str.isdigit()). If the input is not all digits, you can use
exit('error message') to exit the program.
Sample program runs:
$ python ex2.8.py
how many times should I greet you?
error: please enter an integer
three
# i.e., exit('error: please...')
$ python ex2.8.py
how many times should I greet you?
Happy birthday to you!
Happy birthday to you!
Happy birthday to you!
3
$ python ex2.8.py
how many times should I greet you?
Happy birthday to you!
Happy birthday to you!
2
Ex. 2.8 Use the string count() method to count the number of times 'or' appears in the
following text: I am happy or sad or angry or mad or generous or stingy.
Starter code: start your program with this code
msg = 'I am happy or sad or angry or mad or generous or stingy.'
Expected output:
$ python ex2.9.py
5
Ex. 2.9 Modifying the above program, take user input of a string to count, and then report
the number of occurrences in the sentence.
Starter code: same as above
Sample program runs:
$ python ex2.10.py
please enter a string to search:
5
or
$ python ex2.10.py
please enter a string to search:
2
m
$ python ex2.10.py
please enter a string to search:
0
x
Ex. 2.10 Taking a user input substring, replace the substring in the string with 'x's
Starter code: same as above
Sample program runs:
$ python ex2.12.py
I am happy or sad or angry or mad or generous or stingy.
please enter a character to replace: a
I xm hxppy or sxd or xngry or mxd or generous or stingy.
$ python ex2.12.py
I am happy or sad or angry or mad or generous or stingy.
please enter a character to replace: or
I am happy x sad x angry x mad x generous x stingy.
HOMEWORK
Feel free to refer to the discussion document for further detail on how to proceed (including a
code outline).
2.1
Count from 0 up to 100 by 2's (or 3's, or 4's…). Use a while True: loop and
input() to take one input from the user: an integer "step" value. Test to ensure
that the value is an integer value, and keep taking input until correct. Then print
out all the values from 0 up to 100 (or less than 100 if the step value doesn't divide
evenly into 100).
Sample program runs:
$ python homework_2-1.py
please enter an integer: x
sorry, 'x' is not a valid integer
please enter an integer: 3
counting from 0 to 100 by 3's
0
3
6
9
12
15
... (etc., up to 99)
$ python homework_2-1.py
please enter an integer: 25
counting from 0 to 100 by 25's
0
25
50
75
100
2.2
Sum a sequence of integers. As in previous, use a while True: loop to take a
positive integer value (i.e., reject non-integers as well as values < 1. Now sum up
all integers between 0 and the submitted value.
Sample program runs (user input in bold):
$ python dblaikie_2.2.py
please enter a positive integer: hey
sorry, 'hey' is not a valid positive integer
please enter a positive integer: 0
sorry, '0' is not a valid positive integer
please enter a positive integer: 1
sum from 1 to 1 is 1
2.3
$ python dblaikie_2.2.py
please enter a positive integer:
sum from 1 to 2 is 3
2
$ python dblaikie_2.2.py
please enter a positive integer:
sum from 1 to 3 is 6
3
$ python dblaikie_2.2.py
please enter a positive integer:
sum from 1 to 4 is 10
4
$ python dblaikie_2.2.py
please enter a positive integer:
sum from 1 to 5 is 15
5
Text replacement utility. Complete this program so that it can replace any word or
phrase in a text file, and then output the file text after the replacements have been
made. It can also report the number of replacements that were made.
Take two strings using input(): target text and replace text. The supplied starter
code will open and read the file out to a string; complete the program by replacing
the target text with the replace text, and then report on the number of replacements
made.
If you would like to apply this to a file, comment out the last line in this sample and
uncomment the line before it. Add an input() statement to the other two that
prompts for the filename.
Sample run:
python homework_2-3.py
please enter search text: yourself
please enter replace text: your Corgi
And since you know you cannot see your Corgi,
so well as by reflection, I, your glass,
will modestly discover to your Corgi,
that of your Corgi which you yet know not of.
3 replacements made.
Starter code:
sample_text = """And since you know you cannot see yourself,
so well as by reflection, I, your glass,
will modestly discover to yourself,
that of yourself which you yet know not of."""
def read_file(fname):
try:
text = open(fname).read()
except IOError:
print('file {} not found or could not be read.
print('Using sample text instead.')
text = sample_text
return text
'.format(fname))
# to use a file instead of sample text, add a comment mark (#)
# to start of next line, and remove comment mark from the line after
# text = read_file(input('please enter a filename:
text = sample_text
# [YOUR CODE GOES HERE]
[continued]
'))
SUPPLEMENTARY (EXTRA CREDIT) EXERCISES
2.4
Reverse Number Guessor. Create a program that selects a random number
between 1 and 100 (see starter code below), but keeps it secret. It then tells the
user whether his or her guess is correct, is greater than or is less than the user's
guess.
Starting Code:
import random
number_to_guess = random.randint(1,100)
Sample program run (user input in bold):
I am thinking of a number from 1 to 100.
you hints about it.
Try to guess it and I'll give
Your guess? (q to quit): 50
your guess is LOWER than the number I'm thinking of.
Your guess? (q to quit): 75
your guess is LOWER than the number I'm thinking of.
Your guess? (q to quit): 87
your guess is HIGHER than the number I'm thinking of.
Your guess?
you got it!
2.5
(q to quit): 81
You guessed it in 4 tries.
Number Guessor. Write a program that attempts to guess a number from 1 to 100
that the user has chosen and is keeping secret. To guess the number, the
program asks the user a series of guesses and questions, narrowing down the
possibilities by half each time. The program will:
a.
b.
c.
d.
e.
f.
Ask the user to think of a number from 1 to 100
Ask the user if the number is 50 (i.e., halfway between 1 and 100
If the user answers "no", ask the user if the number is higher or lower than 50
If lower, ask the user if the number is 25 (i.e., halfway between 1 and 50)
If higher, ask the user if the number is 75 (i.e., halfway between 50 and 100)
Return to question c -- if the guess is incorrect, ask the user if the number is
higher or lower than 25 (or 75)
g. Continue halving the possibiltiies and guessing the number until the user
reports that the guess is correct.
[continued]
Sample program run (user input in bold):
Think of a number between 100 and 0, and I will try to guess it.
[Enter] to start.
is it 50 (yes/no/quit)? no
is it higher or lower than 50? higher
is it 76 (yes/no/quit)? no
is it higher or lower than 76? lower
is it 63 (yes/no/quit)? no
is it higher or lower than 63? lower
is it 57 (yes/no/quit)? no
is it higher or lower than 57? lower
is it 54 (yes/no/quit)? no
is it higher or lower than 54? lower
is it 52 (yes/no/quit)? no
is it higher or lower than 52? lower
is it 51 (yes/no/quit)? yes
I knew I could do it!
2.6
Hit
Find prime numbers in a range of numbers. Ask the user for an integer, and
display all prime numbers between 2 and the user's number.
Sample program run (user input in bold):
$ python primes.py
* Prime Numbers *
Please enter max integer:
2
3
5
5
$ python primes.py
* Prime Numbers *
Please enter max integer:
2
3
5
7
11
13
19
2.7
20
Chicken, Fox and Grain game. This is rather lengthy assignment using while,
break and continue, as well as numerous if tests comparing strings and numbers.
(In fact, it requires a great deal more if/then/else than might be needed if we were
to use some upcoming Python features. But, it provides an interesting challenge
using this week's features.)
The program is an implementation of a well-known logic puzzle:
A man has to transport a fox, a chicken and a sack of corn across a river using a
rowboat, but the rowboat has room for only one of the items besides himself.
Therefore when crossing the river he usually has to leave two of the items alone on
one of the shores. The problem is that if he leaves the fox alone with the chicken,
the fox will eat the chicken, and if he leaves the chicken alone with the grain, the
chicken will eat the grain. How can he move all three items across the river
without losing any of them?
Sample program runs (user input in bold):
$ python david_blaikie_2.7.py
**THE FOX, THE CHICKEN AND THE GRAIN**
You are on the east bank of a river.
You must transport a fox, a chicken
and a bag of grain to the west bank.
However, you can only carry one item
at a time. Good luck!
you
the
the
the
now are on the east bank.
chicken is on the east bank.
fox is on the east bank.
grain is on the east bank.
What would you carry in the seat of the boat ([Enter] for nothing, q to
quit)? xx
sorry, I don't recognize 'xx'.
you
the
the
the
Try again.
now are on the east bank.
chicken is on the east bank.
fox is on the east bank.
grain is on the east bank.
What would you carry in the seat of the boat ([Enter] for nothing, q to
quit)?
What would you carry in the seat of the boat ([Enter] for nothing, q to
quit)? grain
oops, the fox got the chicken!
$ python david_blaikie_2.7.py
**THE FOX, THE CHICKEN AND THE GRAIN**
You are on the east bank of a river.
You must transport a fox, a chicken
and a bag of grain to the west bank.
However, you can only carry one item
at a time. Good luck!
you
the
the
the
now are on the east bank.
chicken is on the east bank.
fox is on the east bank.
grain is on the east bank.
What would you carry in the seat of the boat ([Enter] for nothing, q to
quit)? fox
oops, the chicken got the grain!
$ python david_blaikie_2.7.py
**THE FOX, THE CHICKEN AND THE GRAIN**
You are on the east bank of a river.
You must transport a fox, a chicken
and a bag of grain to the west bank.
However, you can only carry one item
at a time. Good luck!
you
the
the
the
now are on the east bank.
chicken is on the east bank.
fox is on the east bank.
grain is on the east bank.
What would you carry in the seat of the boat ([Enter] for nothing, q to
quit)? chicken
...carrying the chicken to the west bank...
you
the
the
the
now are on the west bank.
chicken is on the west bank.
fox is on the east bank.
grain is on the east bank.
What would you carry in the seat of the boat ([Enter] for nothing, q to
quit)? [pressed Enter, meaning take back nothing]
...traveling to the east bank...
you
the
the
the
now are on the east bank.
chicken is on the west bank.
fox is on the east bank.
grain is on the east bank.
What would you carry in the seat of the boat ([Enter] for nothing, q to
quit)? fox
...carrying the fox to the west bank...
you
the
the
the
now are on the west bank.
chicken is on the west bank.
fox is the west bank.
grain is on the east bank.
What would you carry in the seat of the boat ([Enter] for nothing, q to
quit)? [pressed Enter, meaning take back nothing]
oops, the fox got the chicken!